Here is the original paper on DTLS, the security basis for CoAP/UDP from Nagendra Modadugu and Eric Rescorla:
Applications that are based on reliable transport can be secured using TLS, but no compelling alternative exists for securing datagram-based applications. In this paper we present DTLS, a datagram-capable version of TLS.
Well written.
About Mark Benson
Mark Benson is Head of Samsung SmartThings, author of The Art of Software Thermal Management for Embedded Systems, and is a regular speaker and writer on leadership, organizational behavior, and the future of the smart home.